1. Field of the Invention
This invention relates to an apparatus for, and a method of, recovery of metal from solution, employing an electrolytic cell. The invention finds particular, though not exclusive application in the recovery of silver from photographic processing solutions, especially from the fixing stage thereof.
2. Description of the Related Art
For convenience and by way of example only, the invention will be described with reference to black and white photographic processing solutions and the recovery of silver therefrom.
Photographic material, in sheet or roll film form, is processed in several stages, including undergoing chemical development, fixing of the image, washing and drying. The role of the photographic fixing solution is to form soluble salts of any unexposed silver halide grains in the emulsion of the sensitized material. As more film is processed, the fixing solution becomes seasoned with soluble silver ion complexes. These complexes reduce the ability of the solution to fix the image, and thus affect its final quality. Ultimately, in some instances the solution could become too loaded with silver and it would be necessary to replace it with a totally fresh solution. However, environmental legislation is increasingly putting stricter limitations on the disposal of waste material bearing silver. Consequently, attention is increasingly being paid to safe and efficient recovery of the silver, and it is known to do this electrolytically. The advantages of in-line electrolytic recovery of the silver include:
(i) the lifetime of the fixing solution can be extended, PA1 (ii) the rate of fixing of the image can be increased, PA1 (iii) the rate of replenishment of the solution with fresh chemicals can be reduced, PA1 (iv) treatment of the effluent from the photographic processing is facilitated, PA1 (v) the value of the silver recovered is economically worthwhile; and PA1 (vi) reduced carryover of silver into the wash, with consequent lower silver concentration in the wash effluent. PA1 In the electrolytic cell, silver is deposited on the cathode, and when this is full of silver action must be taken by the user of the equipment, and there are two options. The first option is when the cathode is reusable, in which case it can be taken out of the cell, the silver removed and the cathode then replaced. However, this is a messy and inconvenient operation for the user and can result in undesirable contact not only with the silver, but also with the processing solution since the cathode has been immersed in it. The other option arises when the cathode is a disposable one, whereby it is simply taken out of the cell and replaced with a fresh one. The fully-laden cathode can then be sent to a refiner who will put both the silver and the cathode into a smelting process. This is applicable when the cathode is made of a material which is both low-cost and compatible with the refining process.
Examples of such cathodes are those in which a plastics material is laminated to graphite or coated with conductive ink. Although this type of operation is less messy and inconvenient than that of the reusable cathode, it still can lead to the user having contact with the processing solution. The laden cathode is wet and the changeover operation may involve draining the cell or operating various taps and valves. Replacing the cathode may involve refilling the cell and bleeding the air out of it.
One example of an electrolytic cell is disclosed in CH-A-647005, in which there is an inlet opening at the base of the closed cylindrical cell through which solution is introduced tangentially. The outlet is at the other end (the top) of the cylinder at a tangent, or it is located in the middle. The outlet opening is two to four times the diameter of the inlet opening.
U.S Pat. No. 4,280,884 discloses a closed cylindrical electrolytic cell in which the cathode is attached to the lid of the cell and is reusable. After removing the lid together with the cathode from the cell, the cathode has to be detached from the lid and the silver scraped off before it is replaced in the cell. Solution flows into the cell from the base of a hollow tubular anode and out through a plurality of holes in the wall thereof, causing jets of liquid to be directed towards the cathode, which is in the form of a cylinder close to the outer wall of the cell. The solution leaves the cell from an outlet pipe towards the top thereof.
U.S. Pat. No. 4,372,829 also discloses a closed cylindrical electrolytic cell in which the cathode is attached to the lid. Solution flow into and out of the cell is arranged to be through the base thereof, and the lid is supplied with an air bleed valve. Each time the cathode is removed, air must be evacuated by the user through the bleed valve. The requirement for bleeding of the cell is inconvenient, requires a degree of operator skill, and is prone to leakage. The flow profile through the cell is such that the silver-laden fixer may reside therein for a time such that sulphiding, that is to say the formation of silver sulphide as a fine precipitate in the solution, can occur.
U.S. Pat. No. 5,370,781 discloses a closed cylindrical electrolytic cell employing a disposable cathode. A cathode is inserted into a tubular casing and when the lid is screwed down from above, electrical contact is made as the cathode is pressed against a contact point in the wall of the casing. To change the cathode, flow taps must be closed manually, the lid must be removed and the cathode gripped, either by hand or with a retracting tool, to remove it. Solution flows into the cell at the base, up through the cylinder formed by the cathode, through holes in the top of the cathode, and out of the cell through an upper port. An air gap is maintained at the top of the cell above the port to prevent contact between the solution and the connecting ring for the cathode, which would otherwise cause corrosion. Since the cathode has to be apertured for the flow of liquid therethrough, the area available for silver plating is reduced, thus reducing the silver capacity of the cathode, and leading to an increase in the cathode current density for a given recovery rate.
U.S. Pat. No. 5,017,273 discloses a cell containing a disposable cathode, in which the cylindrical cell body and the base are integrally moulded, with the cathode being mechanically fixed to the inner wall of the body. Solution is injected into the cell at an angle to the radius thereof, inducing a vortex, high agitation flow. The solution leaves the cell from an outlet towards the top thereof. In a first embodiment, a "double-container" configuration is employed that avoids the need for air bleeding and for a special drain operation during changeover of the cathode. However, since drainage of the solution is achieved under gravity through a small drain hole, it is possible for the user to remove the cell lid before the cell has been fully drained.
Furthermore, the air space that exists in this arrangement can allow oxidation reactions to occur with consequent disadvantages for the further processing of the photographic material. The space requirements and cost involved in the "double-container" configuration are also disadvantageous. A "single-container" arrangement is also disclosed. However, when it is required to change the cathode, the user has to empty out the trapped solution, which is a messy and inconvenient operation.